Abstract: Wall thickness asymmetry is a common problem for cross wedge rolling (CWR) hollow shafts, which is especially usual in the CWR process of hollow shafts of small diameter and large height-diameter ratio. In this paper, the constitutive equation of hot deformation of 5Cr21Mn9Ni4N heat resistant steel was investigated by the thermocompression experiment on the Gleeble-1500D thermo-simulation machine. The law for the variation of wall thickness during forming 5Cr21Mn9Ni4 hollow valve with CWR was presented, which is obtained via changing mandrel diameter and by means of finite element method (FEM) and experiment. The results confirm that when hollow valve with mandrel is formed with the CWR process, there is a critical diameter of mandrel. It makes wall thickness of hollow valve optimal. On the basis of FEM, it is stated that uniform flow of material along the axial direction is necessary for wall thickness uniformity of hollow valve, and circumferential stretching strain near zero, increase of radial compression strain with decrease of axial tension strain permit to improve wall thickness uniformity of hollow valve.